Na channels are central to the generation of action potentials in all excitable cells such as neurons and myocytes. They play key roles in excitable tissue including brain, smooth muscles of the gastrointestinal tract, skeletal muscle, the peripheral nervous system, spinal cord and airway. As such they play key roles in a variety of disease states such as epilepsy (See, Moulard, B. and D. Bertrand (2002) “Epilepsy and sodium channel blockers” Expert Opin. Ther. Patents, 12(1): 85-91)), pain (See, Waxman, S. G., S. Dib-Hajj, et al. (1999) “Sodium channels and pain” Proc. Natl. Acad. Sci. USA, 96(14): 7635-9 and Waxman, S. G., T. R. Cummins, et al. (2000) “Voltage-gated sodium channels and the molecular pathogenesis of pain: a review” J Rehabil. Res. Dev., 37(5): 517-28), myotonia (See, Meola, G. and V. Sansone (2000) “Therapy in myotonic disorders and in muscle channelopathies” Neurol. Sci., 21(5): S953-61 and Mankodi, A. and C. A. Thornton (2002) “Myotonic syndromes” Curr. Opin. Neurol., 15(5): 545-52), ataxia (See, Meisler, M. H., J. A. Kearney, et al. (2002) “Mutations of voltage-gated sodium channels in movement disorders and epilepsy” Novartis Found. Symp., 241: 72-81), multiple sclerosis (See, Black, J. A., S. Dib-Hajj, et al. (2000) “Sensory neuron-specific sodium channel SNS is abnormally expressed in the brains of mice with experimental allergic encephalomyelitis and humans with multiple sclerosis” Proc. Natl. Acad. Sci. USA, 97(21): 11598-602, and Renganathan, M., M. Gelderblom, et al. (2003) “Expression of Na(v)1.8 sodium channels perturbs the firing patterns of cerebellar purkinje cells” Brain Res., 959(2): 235-42), irritable bowel (See Su, X., R. E. Wachtel, et al. (1999) “Capsaicin sensitivity and voltage-gated sodium currents in colon sensory neurons from rat dorsal root ganglia” Am. J. Physiol., 277(6 Pt 1): G1180-8, and Laird, J. M., V. Souslova, et al. (2002) “Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice” J. Neurosci., 22(19): 8352-6), urinary incontinence and visceral pain (See, Yoshimura, N., S. Seki, et al. (2001) “The involvement of the tetrodotoxin-resistant sodium channel Na(v)1.8 (PN3/SNS) in a rat model of visceral pain” J. Neurosci., 21(21): 8690-6), as well as an array of psychiatry dysfunctions such as anxiety and depression (See, Hurley, S. C. (2002) “Lamotrigine update and its use in mood disorders” Ann. Pharmacother., 36(5): 860-73).
Voltage gated Na channels comprise a gene family consisting of 9 different subtypes (NaV1.1-NaV1.9). As shown in Table 1, these subtypes show tissue specific localization and functional differences (See, Goldin, A. L. (2001) “Resurgence of sodium channel research” Annu. Rev. Physiol., 63: 871-94). Three members of the gene family (NaV1.8, 1.9, 1.5) are resistant to block by the well-known Na channel blocker TTX, demonstrating subtype specificity within this gene family. Mutational analysis has identified glutamate 387 as a critical residue for TTX binding (See, Noda, M., H. Suzuki, et al. (1989) “A single point mutation confers tetrodotoxin and saxitoxin insensitivity on the sodium channel II” FEBS Lett., 259(1): 213-6).
TABLE 1(Abbreviations: CNS = central nervous system,PNS = peripheral nervous sytem, DRG = dorsalroot ganglion, TG = Trigeminal ganglion):NaisoformTissueTTX IC50IndicationsNaV1.1CNS, PNS10nMPain, Epilepsy,soma ofneurodegenerationneuronsNaV1.2CNS, high in10nMNeurodegenerationaxonsEpilepsyNaV1.3CNS,15nMPainembryonic,injured nervesNaV1.4Skeletal25nMMyotoniamuscleNaV1.5Heart2μMArrythmia,long QTNaV1.6CNS6nMPain, movement disorderswidespread,most abuntantNaV1.7PNS, DRG,25nMPain, NeuroendocrineterminalsdisordersneuroendocrineNaV1.8PNS, small>50μMPainneurons inDRG & TGNaV1.9PNS, small1μMPainneurons inDRG & TG
In general, voltage-gated sodium channels (NaVs) are responsible for initiating the rapid upstroke of action potentials in excitable tissue in nervous system, which transmit the electrical signals that compose and encode normal and aberrant pain sensations. Antagonists of NaV channels can attenuate these pain signals and are useful for treating a variety of pain conditions, including but not limited to acute, chronic, inflammatory, and neuropathic pain. Known NaV antagonists, such as TTX, lidocaine (See, Mao, J. and L. L. Chen (2000) “Systemic lidocaine for neuropathic pain relief” Pain 87(1): 7-17) bupivacaine, phenytoin (See, Jensen, T. S. (2002) “Anticonvulsants in neuropathic pain: rationale and clinical evidence” Eur. J. Pain, 6 (Suppl A): 61-8), lamotrigine (See Rozen, T. D. (2001) “Antiepileptic drugs in the management of cluster headache and trigeminal neuralgia” Headache, 41 Suppl 1: S25-32 and Jensen, T. S. (2002) “Anticonvulsants in neuropathic pain: rationale and clinical evidence” Eur. J. Pain, 6 (Suppl A): 61-8), and carbamazepine (See, Backonja, M. M. (2002) “Use of anticonvulsants for treatment of neuropathic pain” Neurology 59(5 Suppl 2): S14-7), have been shown to be useful attenuating pain in humans and animal models.
Hyperalgesia (extreme sensitivity to something painful) that develops in the presence of tissue injury or inflammation reflects, at least in part, an increase in the excitability of high-threshold primary afferent neurons innervating the site of injury. Voltage sensitive sodium channels activation is critical for the generation and propagation of neuronal action potentials. There is a growing body of evidence indicating that modulation of NaV currents is an endogenous mechanism used to control neuronal excitability (See Goldin, A. L. (2001) “Resurgence of sodium channel research” Annu. Rev. Physiol., 63: 871-94). Several kinetically and pharmacologically distinct voltage-gated sodium channels are found in dorsal root ganglion (DRG) neurons. The TTX-resistant current is insensitive to micromolar concentrations of tetrodotoxin, and displays slow activation and inactivation kinetics and a more depolarized activation threshold when compared to other voltage-gated sodium channels. TTX-resistant sodium currents are primarily restricted to a subpopulation of sensory neurons likely to be involved in nociception. Specifically, TTX-resistant sodium currents are expressed almost exclusively in neurons that have a small cell-body diameter; and give rise to small-diameter slow-conducting axons and that are responsive to capsaicin. A large body of experimental evidence demonstrates that TTX-resistant sodium channels are expressed on C-fibers and are important in the transmission of nociceptive information to the spinal cord.
Intrathecal administration of antisense oligo-deoxynucleotides targeting a unique region of the TTX-resistant sodium channel (NaV1.8) resulted in a significant reduction in PGE2-induced hyperalgesia (See, Khasar, S. G., M. S. Gold, et al. (1998) “A tetrodotoxin-resistant sodium current mediates inflammatory pain in the rat” Neurosci. Lett., 256(1): 17-20). More recently, a knockout mouse line was generated by Wood and colleagues, which lacks functional NaV1.8. The mutation has an analgesic effect in tests assessing the animal's response to the inflammatory agent carrageenan (See, Akopian, A. N., V. Souslova, et al. (1999) “The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways” Nat Neurosci., 2(6): 541-8). In addition, deficit in both mechano- and thermoreception were observed in these animals. The analgesia shown by the Nav1.8 knockout mutants is consistent with observations about the role of TTX-resistant currents in nociception.
Immunohistochemical, in-situ hybridization and in-vitro electrophysiology experiments have all shown that the sodium channel NaV1.8 is selectively localized to the small sensory neurons of the dorsal root ganglion and trigeminal ganglion (See, Akopian, A. N., L. Sivilotti, et al. (1996) “A tetrodotoxin-resistant voltage-gated sodium channel expressed by sensory neurons” Nature, 379(6562): 257-62). The primary role of these neurons is the detection and transmission of nociceptive stimuli. Antisense and immunohistochemical evidence also supports a role for NaV1.8 in neuropathic pain (See, Lai, J., M. S. Gold, et al. (2002) “Inhibition of neuropathic pain by decreased expression of the tetrodotoxin-resistant sodium channel, NaV1.8” Pain 95(1-2): 143-52, and Lai, J., J. C. Hunter, et al. (2000) “Blockade of neuropathic pain by antisense targeting of tetrodotoxin-resistant sodium channels in sensory neurons” Methods Enzymol., 314: 201-13). NaV1.8 protein is upregulated along uninjured C-fibers adjacent to the nerve injury. Antisense treatment prevents the redistribution of NaV1.8 along the nerve and reverses neuropathic pain. Taken together the gene-knockout and antisense data support a role for NaV1.8 in the detection and transmission of inflammatory and neuropathic pain.
In neuropathic pain states there is a remodeling of Na channel distribution and subtype. In the injured nerve, expression of NaV1.8 and NaV1.9 are greatly reduced whereas expression of the TTX sensitive subunit NaV1.3 is 5-10 fold upregulated (See, Dib-Hajj, S. D., J. Fjell, et al. (1999) “Plasticity of sodium channel expression in DRG neurons in the chronic constriction injury model of neuropathic pain.” Pain, 83(3): 591-600.) The timecourse of the increase in NaV1.3 parallels the appearance of allodynia in animal models subsequent to nerve injury. The biophysics of the NaV1.3 channel is distinctive in that it shows very fast repriming after inactivation following an action potential. This allows for sustained rates of high firing as is often seen in the injured nerve (See, Cummins, T. R., F. Aglieco, et al. (2001) “Nav1.3 sodium channels: rapid repriming and slow closed-state inactivation display quantitative differences after expression in a mammalian cell line and in spinal sensory neurons” J. Neurosci., 21(16): 5952-61.). NaV1.3 is expressed in the central and peripheral systems of man. NaV1.9 is similar to NaV1.8 as it is selectively localized to small sensory neurons of the dorsal root ganglion and trigeminal ganglion (See, Fang, X., L. Djouhri, et al. (2002). “The presence and role of the tetrodotoxin-resistant sodium channel Na(v)1.9 (NaN) in nociceptive primary afferent neurons.” J. Neurosci., 22(17): 7425-33.). It has a slow rate of inactivation and left-shifted voltage dependence for activation (See, Dib-Hajj, S., J. A. Black, et al. (2002) “NaN/Nav1.9: a sodium channel with unique properties” Trends Neurosci., 25(5): 253-9.). These two biophysical properties allow NaV1.9 to play a role in establishing the resting membrane potential of nociceptive neurons. The resting membrane potential of NaV1.9 expressing cells is in the −55 to −50 mV range compared to −65 mV for most other peripheral and central neurons. This persistent depolarization is in large part due to the sustained low-level activation of NaV1.9 channels. This depolarization allows the neurons to more easily reach the threshold for firing action potentials in response to nociceptive stimuli. Compounds that block the NaV1.9 channel may play an important role in establishing the set point for detection of painful stimuli. In chronic pain states, nerve and nerve ending can become swollen and hypersensitive exhibiting high frequency action potential firing with mild or even no stimulation. These pathologic nerve swellings are termed neuromas and the primary Na channels expressed in them are NaV1.8 and NaV1.7 (See, Kretschmer, T., L. T. Happel, et al. (2002) “Accumulation of PN1 and PN3 sodium channels in painful human neuroma-evidence from immunocytochemistry” Acta Neurochir. (Wien), 144(8): 803-10; discussion 810.). NaV1.6 and NaV1.7 are also expressed in dorsal root ganglion neurons and contribute to the small TTX sensitive component seen in these cells. NaV1.7 in particular my therefore be a potential pain target in addition to it's role in neuroendocrine excitability (See, Klugbauer, N., L. Lacinova, et al. (1995) “Structure and functional expression of a new member of the tetrodotoxin-sensitive voltage-activated sodium channel family from human neuroendocrine cells” Embo J., 14(6): 1084-90).
NaV1.1 (See, Sugawara, T., E. Mazaki-Miyazaki, et al. (2001) “Nav1.1 mutations cause febrile seizures associated with afebrile partial seizures.” Neurology, 57(4): 703-5.) and NaV1.2 (See, Sugawara, T., Y. Tsurubuchi, et al. (2001) “A missense mutation of the Na+ channel alpha II subunit gene Na(v)1.2 in a patient with febrile and afebrile seizures causes channel dysfunction” Proc. Natl. Acad. Sci. USA, 98(11): 6384-9) have been linked to epilepsy conditions including febrile seizures. There are over 9 genetic mutations in NaV1.1 associated with febrile seizures (See, Meisler, M. H., J. A. Kearney, et al. (2002) “Mutations of voltage-gated sodium channels in movement disorders and epilepsy” Novartis Found. Symp. 241: 72-81)
Antagonists for NaV1.5 have been developed and used to treat cardiac arrhythmias. A gene defect in NaV1.5 that produces a larger noninactivating component to the current has been linked to long QT in man and the orally available local anesthetic mexilitine has been used to treat this condition (See, Wang, D. W., K. Yazawa, et al. (1997) “Pharmacological targeting of long QT mutant sodium channels.” J. Clin. Invest., 99(7): 1714-20).
Several Na channel blockers are currently used or being tested in the clinic to treat epilepsy (See, Moulard, B. and D. Bertrand (2002) “Epilepsy and sodium channel blockers” Expert Opin. Ther. Patents, 12(1): 85-91); acute (See, Wiffen, P., S. Collins, et al. (2000) “Anticonvulsant drugs for acute and chronic pain” Cochrane Database Syst, Rev, 3, chronic (See, Wiffen, P., S. Collins, et al. (2000) “Anticonvulsant drugs for acute and chronic pain” Cochrane Database Syst. Rev., 3, and Guay, D. R. (2001) “Adjunctive agents in the management of chronic pain” Pharmacotherapy, 21(9): 1070-81), inflammatory (See, Gold, M. S. (1999) “Tetrodotoxin-resistant Na+ currents and inflammatory hyperalgesia.” Proc. Natl. Acad. Sci. USA, 96(14): 7645-9), and neuropathic pain (See, Strichartz, G. R., Z. Zhou, et al. (2002) “Therapeutic concentrations of local anaesthetics unveil the potential role of sodium channels in neuropathic pain” Novartis Found. Symp., 241: 189-201, and Sandner-Kiesling, A., G. Rumpold Seitlinger, et al. (2002) “Lamotrigine monotherapy for control of neuralgia after nerve section” Acta Anaesthesiol. Scand., 46(10): 1261-4); cardiac arrhythmias (See, An, R. H., R. Bangalore, et al. (1996) “Lidocaine block of LQT-3 mutant human Na+ channels” Circ. Res., 79(1): 103-8, and Wang, D. W., K. Yazawa, et al. (1997) “Pharmacological targeting of long QT mutant sodium channels” J. Clin. Invest., 99(7): 1714-20); neuroprotection (See, Taylor, C. P. and L. S. Narasimhan (1997) “Sodium channels and therapy of central nervous system diseases” Adv. Pharmacol., 39: 47-98) and as anesthetics (See, Strichartz, G. R., Z. Zhou, et al. (2002) “Therapeutic concentrations of local anaesthetics unveil the potential role of sodium channels in neuropathic pain.” Novartis Found. Symp., 241: 189-201).
Various animal models with clinical significance have been developed for the study of sodium channel modulators for numerous different pain indications. E.g., malignant chronic pain, see, Kohase, H., et al., Acta Anaesthesiol Scand., 2004; 48(3):382-3; femur cancer pain (see, Kohase, H., et al., Acta Anaesthesiol Scand., 2004; 48(3):382-3); non-malignant chronic bone pain (see, Ciocon, J. O. et al., J. Am. Geriatr. Soc., 1994; 42(6):593-6); rheumatoid arthritis (see, Calvino, B. et al., Behav. Brain Res., 1987; 24(1):11-29); osteoarthritis (see, Guzman, R. E., et al., Toxicol. Pathol., 2003; 31(6):619-24); spinal stenosis (see, Takenobu, Y. et al., J. Neurosci. Methods, 2001; 104(2):191-8); neuropathic low back pain (see, Hines, R., et al., Pain Med., 2002; 3(4):361-5; Massie, J. B., et al., J. Neurosci. Methods, 2004; 137(2):283-9); myofascial pain syndrome (see, Dalpiaz & Dodds, J. Pain Palliat. Care Pharmacother., 2002; 16(1):99-104; Sluka, K. A. et al., Muscle Nerve, 2001; 24(1):37-46); fibromyalgia (see, Bennet & Tai, Int. J. Clin. Pharmacol. Res., 1995;15(3):115-9); temporomandibular joint pain (see, Ime H., Ren K., Brain Res. Mol. Brain Res., 1999; 67(1):87-97); chronic visceral pain, including, abdominal (see, Al-Chaer, E. D., et al., Gastroenterology, 2000; 119(5):1276-85; pelvic/perineal (see, Wesselmann et al., Neurosci. Lett., 1998; 246(2):73-6); pancreatic (see, Vera-Portocarrero, L. B., et al., Anesthesiology, 2003; 98(2):474-84); IBS pain (see, Verne, G. N., et al., Pain, 2003; 105(1-2):223-30; La J. H. et al., World Gastroenterol., 2003; 9(12):2791-5); chronic headache pain (see, Willimas & Stark, Cephalalgia, 2003; 23(10):963-71); migraine (see, Yamamura, H., et al., J. Neurophysiol., 1999; 81(2):479-93); tension headache, including, cluster headaches (see, Costa, A., et al., Cephalalgia, 2000; 20(2):85-91); chronic neuropathic pain, including, post-herpetic neuralgia (see, Attal, N., et al., Neurology, 2004; 62(2):218-25; Kim & Chung 1992, Pain, 50:355); diabetic neuropathy (see, Beidoun A et al., Clin. J. Pain., 2004; 20(3):174-8; Courteix, C., et al., Pain, 1993; 53(1):81-8); HIV-associated neuropathy (see, Portegies & Rosenberg, Ned. Tijdschr. Geneeskd., 2001; 145(15):731-5; Joseph, E. K. et al., Pain, 2004; 107(1-2):147-58; Oh, S. B., et al., J. Neurosci., 2001; 21(14):5027-35); trigeminal neuralgia (see, Sato, J., et al., Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod., 2004; 97(1):18-22; Imamura, Y. et al., Exp. Brain Res., 1997; 116(1):97-103); Charcot-Marie Tooth neuropathy (see, Sereda, M., et al., Neuron, 1996; 16(5):1049-60); hereditary sensory neuropathies (see, Lee, M. J., et al., Hum. Mol. Genet., 2003; 12(15):1917-25); peripheral nerve injury (see, Attal, N., et al., Neurology, 2004; 62(2):218-25; Kim & Chung 1992, Pain, 50:355; Bennett & Xie, 1988, Pain., 33:87; Decostered, I. & Woolf, C. J., 2000, Pain, 87:149; Shir, Y. & Seltzer, Z. 1990; Neurosci. Lett., 115:62); painful neuromas (see, Nahabedian & Johnson, Ann. Plast. Surg., 2001; 46(1):15-22; Devor & Raber, Behav. Neural. Biol., 1983; 37(2):276-83); ectopic proximal and distal discharges (see, Liu, X. et al., Brain Res., 2001; 900(1):119-27); radiculopathy (see, Devers & Galer, (see, Clin. J. Pain., 2000; 16(3):205-8; Hayashi, N. et al., Spine, 1998; 23(8):877-85); chemotherapy induced neuropathic pain (see, Aley, K. O., et al., Neuroscience, 1996; 73(1):259-65); radiotherapy-induced neuropathic pain; post-mastectomy pain (see, Devers & Galer, Clin. J. Pain, 2000; 16(3):205-8); central pain (Cahana, A., et al., Anesth. Analg., 2004; 98(6):1581-4), spinal cord injury pain (see, Hains, B. C., et al., Exp. Neurol., 2000; 164(2):426-37); post-stroke pain; thalamic pain (see, LaBuda, C. J., et al., Neurosci. Lett., 2000; 290(1):79-83); complex regional pain syndrome (see, Wallace, M. S., et al., Anesthesiology, 2000; 92(1):75-83; Xantos D et al., J. Pain, 2004; 5(3 Suppl 2):S1); phanton pain (see, Weber, W. E., Ned. Tiidschr. Geneeskd., 2001; 145(17):813-7; Levitt & Heyback, Pain, 1981; 10(1):67-73); intractable pain (see, Yokoyama, M., et al., Can. J. Anaesth., 2002; 49(8):810-3); acute pain, acute post-operative pain (see, Koppert, W., et al., Anesth. Analg. 2004; 98(4):1050-5; Brennan, T. J., et al., Pain, 1996; 64(3):493-501); acute musculoskeletal pain; joint pain (see, Gotoh, S., et al., Ann. Rheum. Dis., 1993; 52(11):817-22); mechanical low back pain (see, Kehl, L. J., et al., Pain. 2000; 85(3):333-43); neck pain; tendonitis; injury/exercise pain (see, Sesay, M., et al., Can. J. Anaesth., 2002; 49(2):137-43); acute visceral pain, including, abdominal pain; pyelonephritis; appendicitis; cholecystitis; intestinal obstruction; hernias; etc (see, Giambernardino, M. A., et al., Pain, 1995; 61(3):459-69); chest pain, including, cardiac pain (see, Vergona, R. A., et al., Life Sci., 1984; 35(18):1877-84); pelvic pain, renal colic pain, acute obstetric pain, including, labor pain (see, Segal, S., et al., Anesth. Analg., 1998; 87(4):864-9); cesarean section pain; acute inflammatory, burn and trauma pain; acute intermittent pain, including, endometriosis (see, Cason, A. M., et al., Horm. Behav., 2003; 44(2):123-31); acute herpes zoster pain; sickle cell anemia; acute pancreatitis (see, Toma, H.; Gastroenterology, 2000; 119(5):1373-81); breakthrough pain; orofacial pain, including, sinusitis pain, dental pain (see, Nusstein, J., et al., J. Endod., 1998; 24(7):487-91; Chidiac, J. J., et al., Eur. J. Pain., 2002; 6(1):55-67); multiple sclerosis (MS) pain (see, Sakurai & Kanazawa, J. Neurol. Sci., 1999; 162(2):162-8); pain in depression (see, Greene, B., Curr. Med. Res. Opin., 2003; 19(4):272-7); leprosy pain; Behcet's disease pain; adiposis dolorosa (see, Devillers & Oranje, Clin. Exp. Dermatol., 1999; 24(3):240-1); phlebitic pain; Guillain-Barre pain; painful legs and moving toes; Haglund syndrome; erythromelalgia pain (see, Legroux-Crespel, E., et al., Ann. Dermatol. Venereol., 2003; 130(4):429-33); Fabry's disease pain (see, Germain, D. P., J. Soc. Biol., 2002; 196(2):183-90); bladder and urogenital disease, including, urinary incontinence (see, Berggren, T., et al., J. Urol., 1993; 150(5 Pt 1):1540-3); hyperactivity bladder (see, Chuang, Y. C., et al., Urology, 2003; 61(3):664-70); painful bladder syndrome (see, Yoshimura, N., et al., J. Neurosci., 2001; 21(21):8690-6); interstitial cyctitis (IC) (see, Giannakopoulos& Campilomatos, Arch. Ital. Urol. Nefrol. Androl., 1992; 64(4):337-9; Boucher, M., et al., J. Urol., 2000; 164(1):203-8); prostatitis (see, Mayersak, J. S., Int. Surg., 1998; 83(4):347-9; Keith, I. M., et al., J. Urol., 2001; 166(1):323-8).
Calcium channels are membrane-spanning, multi-subunit proteins that allow Ca entry from the external milieu and concurrent depolarization of the cell's membrane potential. Traditionally calcium channels have been classified based on their functional characteristics such as low voltage or high voltage activated and their kinetics (L,T,N,P,Q). The ability to clone and express the calcium channel subunits has lead to an increased understanding of the channel composition that produces these functional responses. There are three primary subunit types that make up calcium channels—α1, α2δ, and β. The α1 is the subunit containing the channel pore and voltage sensor, α2 is primarily extracellular and is disulfide linked to the transmembrane δ subunit, β is nonglycosylated subunit found bound to the cytoplasmic region of the α1 subunit of the Ca channel. Currently the various calcium channel subtypes are believed to made up of the following specific subunits:                L-type, comprising subunits α1Cα1Dα1F, or α1S, α2δ and β3a         N-Type, comprising subunits α1B, α2δ, β1b         P-Type, comprising subunits α1A, α2δ, β4a         Q-Type, comprising subunits α1A (splice variant) α2δ, β4a         R-Type, comprising subunits α1E, α2δ, β1b         T-Type, comprising subunits α1G, α1H, or α1I         
Calcium channels play a central role in neurotransmitter release. Ca influx into the presynaptic terminal of a nerve process binds to and produces a cascade of protein-protein interactions (syntaxin 1A, SNAP-25 and synaptotagmin) that ultimately ends with the fusion of a synaptic vesical and release of the neurotransmitter packet. Blockade of the presynaptic calcium channels reduces the influx of Ca and produces a cubic X3 decrease in neurotransmitter release.
The N type Ca channel (CaV2.2) is highly expressed at the presynaptic nerve terminals of the dorsal root ganglion as it forms a synapse with the dorsal horn neurons in lamina I and II. These neurons in turn have large numbers of N type Ca channels at their presynaptic terminals as they synapse onto second and third order neurons. This pathway is very important in relaying pain information to the brain.
Pain can be roughly divided into three different types: acute, inflammatory, and neuropathic. Acute pain serves an important protective function in keeping the organism safe from stimuli that may produce tissue damage. Severe thermal, mechanical, or chemical inputs have the potential to cause severe damage to the organism if unheeded. Acute pain serves to quickly remove the individual from the damaging environment. Acute pain by its very nature generally is short lasting and intense. Inflammatory pain on the other had may last for much longer periods of time and it's intensity is more graded. Inflammation may occur for many reasons including tissue damage, autoimmune response, and pathogen invasion. Inflammatory pain is mediated by an “inflammatory soup” that consists of substance P, histamines, acid, prostaglandin, bradykinin, CGRP, cytokines, ATP, and neurotransmitter release. The third class of pain is neuropathic and involves nerve damage that results in reorganization of neuronal proteins and circuits yielding a pathologic “sensitized” state that can produce chronic pain lasting for years. This type of pain provides no adaptive benefit and is particularly difficult to treat with existing therapies.
Pain, particularly neuropathic and intractable pain is a large unmet medical need. Millions of individuals suffer from severe pain that is not well controlled by current therapeutics. The current drugs used to treat pain include NSAIDS, COX2 inhibitors, opioids, tricyclic antidepressants, and anticonvulsants. Neuropathic pain has been particularly difficult to treat as it does not respond well to opiods until high doses are reached. Gabapentin is currently the favored therapeutic for the treatment of neuropathic pain although it works in only 60% of patients where it shows modest efficacy. The drug is however very safe and side effects are generally tolerable although sedation is an issue at higher doses.
The N type Ca channel has been validated in man by intrathecal infusion of the toxin Ziconotide for the treatment of intractable pain, cancer pain, opioid resistant pain, and neuropathic and severe pain. The toxin has an 85% success rate for the treatment of pain in humans with a greater potency than morphine. An orally available N type Ca channel antagonist would garner a much larger share of the pain market. Ziconotide causes mast cell degranulation and produces dose-dependent central side effects. These include dizziness, nystagmus, agitation, and dysmetria. There is also orthostatic hypotension in some patients at high doses. The primary risk for this target involves the CNS side effects seen with Ziconotide at high dosing. These include dizziness, nystagmus, agitation, and dysmetria. There is also orthostatic hypotension in some patients at high doses. It is believed that this may be due to Ziconotide induced mast cell degranulation and/or its effects on the sympathetic ganglion that like the dorsal root ganglion also expresses the N type Ca channel. Use-dependent compounds that block preferentially in the higher frequency range >10 Hz should be helpful in minimizing these potential side-effect issues. The firing rate in man of the sympathetic efferents is in the 0.3 Hz range. CNS neurons can fire at high frequencies but generally only do so in short bursts of action potentials. Even with the selectivity imparted by use-dependence intrinsic selectivity against the L type calcium channel is still necessary as it is involved in cardiac and vascular smooth muscle contraction.
Unfortunately, as described above, the efficacy of currently used sodium channel blockers and calcium channel blockers for the disease states described above has been to a large extent limited by a number of side effects. These side effects include various CNS disturbances such as blurred vision, dizziness, nausea, and sedation as well more potentially life threatening cardiac arrhythmias and cardiac failure. Accordingly, there remains a need to develop additional Na channel and Ca channel antagonists, preferably those with higher potency and fewer side effects.